1. Field of the Invention
This invention relates generally to the clutch of a variable speed belt drive, and more particularly to a flyweight for use in a clutch that permits adjustment of the flyweight mass and moment of inertia without the need to remove the flyweight from the driving clutch.
2. Description of Related Technology
A variable speed belt drive is a type of transmission commonly used with internal combustion engines developing fifty to two hundred horsepower at shaft speeds of approximately 10,000 revolutions per minute. Such belt drives are commonly used on snowmobiles, and permit operation from low velocities to speeds of over 100 miles per hour. The belt drive typically includes a driving clutch having a shaft which is coaxial with the output shaft of the vehicle's engine. The driving clutch is formed to include a stationary and a fixed sheave, which together define a pulley around which a belt may travel. The belt also includes travel around a driven clutch pulley that transfers the engine's power to the output shaft driving the vehicle.
An example of a variable speed belt drive of this type is disclosed in U.S. Pat. No. 3,939,720, issued to Aaen et al. The effective radius of both the driving pulley and the driven pulley may be varied, and it is the ratio of the driving pulley radius to the driven pulley radius which determines the ratio of engine speed to the output shaft rate of rotation. If the driving pulley radius is small as compared to the driven pulley, the output shaft will turn at a rate that is slower than the engine speed, resulting in a low vehicle speed. As the ratio of the driving to driven pulley radius approaches 1:1, the output shaft speed will be approximately equal to the engine speed, and the vehicle speed will be relatively greater. Finally, as the driving pulley radius becomes greater than the driven pulley radius, an overdrive condition occurs in which the output shaft is turning at a rate which is greater than the crankshaft of the engine.
Ideally, an engine will deliver power in a linear manner and the transmission will deliver all of the available engine power regardless of the vehicle's speed or load. Unfortunately, that is not the case with either real world engines or transmissions. Instead, the typical engine delivers its maximum power over a narrow range or band of high crankshaft speeds, with power falling off measurably on either side of that band. Ideally, the transmission should permit the engine to operate within that band regardless of the load on the engine. Typically, the maximum "power band" has a range on the order of 100 rpm.
In a variable speed belt transmission, the effective radius of both the driven clutch and the driving clutch are variable and can move while the engine is under power. The driven clutch relies on the combination of a pretension spring and a helical torque feedback ramp to exert the required pressure on the movable pulley sheave to maintain the optimum side load on the belt. While the correct design and adjustment of the driven pulley sheaves determines the efficiency of the transmission system by properly transferring to the output shaft the engine power that is made available to it, the driving clutch must control the engine speed and keep the engine operating in the "power band" throughout the entire operating range of the transmission.
The driving clutch varies its effective radius by having a movable sheave that decreases the distance between the tapering sheaves and thus increases its effective radius as the engine speed increases. Movement of the movable sheave occurs because of the force exerted by one or more flyweights that alter their orientation in response to the centripetal force caused by rotation of the engine. The mass of the flyweight and its moment of inertia are critical to establishing operation of the engine within the "power band". If the flyweight is too heavy, light or not properly balanced in its dynamic state, the driving clutch will not be delivering maximum power to the driven clutch, but will instead be operating above or below the "power band".
Further, once the proper flyweights are chosen, variations over time in the engine output, transmission efficiency or vehicle configuration will cause the power band to shift, thereby requiring the replacement of the flyweights. Replacement of the flyweights presents several problems. First, the construction of the movable sheave housing requires that the driving clutch be substantially disassembled in order to replace the flyweights. Second, there is no readily available method of determining what change to the flyweights is required to achieve the desired result. Hence, one of many fixed weights must be inserted and the clutch reassembled. The vehicle must then be test driven in order to determine if the change in flyweights was helpful. Even if that were the case, there is no way of determining if the substitution of flyweights achieved the optimum results, and so additional disassembly, substitution, reassembly and testing is required. The entire process is so time consuming that it is seldom properly performed, with the result that most snowmobiles, for example, are not actually operating within ten percent of their power band. The resulting inefficiency also causes excess fuel and oil consumption.
Finally, the sheer number of manufacturer supplied flyweights makes it unlikely that a complete supply will be on hand when needed. For example, on page 57 of the Clutch Tuning Handbook by Olav Aaen, 1995 edition published by Aaen Performance, 316 Sheridan Road, Racine, Wis. 53403, shows thirty four separate drive clutch weights that are available for a popular commercial unit. These weights are available in tolerances of .+-.1 gram, meaning that a 50 gram weight could weigh less than a stock 49 gram weight. Also, the thirty four weights represent a variety of shapes and thus moments of inertia, all of which must be tried empirically in order to approach, but not necessarily achieve, optimum performance.
The ideal solution to this problem is to adjust, rather than replace, the existing flyweights while they are still in place on the clutch. One attempt at this approach has been made in the Yamaha YPZ clutch, as discussed on page 58 of the Clutch Tuning Handbook. Unfortunately, the Yamaha approach is limited to the addition of washers on a flyweight of fixed shape. The size of the washers is such that only small changes in weight and moment of inertia can be achieved, and the system assumes that the subtraction of mass from the original flyweight will not be desired.